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Rollin’ – Design considerations and realization of a mobile platform for a humanoid upper body

M. Fuchs, Ch. Borst, P. Robuffo Giordano, A. Baumann, E. Kraemer, J. Langwald, R. Gruber, N. Seitz, G. Plank, K. Kunze, R. Burger, F. Schmidt, T. Wimboeck and G. Hirzinger

German Aerospace Center – Institute of and Mechatronics Oberpfaffenhofen, Germany, 2008 Email: [email protected]

Abstract – Research on humanoid robots for use in servicing tasks, e.g. fetching and delivery, attracts steadily more interest. With Rollin’ Justin a mobile robotic system and research platform is presented that allows the implementation and demonstration of sophisticated control algorithms and dexterous manipulation. Important problems of service robotics such as mobile manipulation and strategies for using the increased workspace and redundancy in manipulation task can be studied in detail. This paper gives an overview of the design considerations for a mobile platform and their realizations to transform the formerly table-mounted humanoid upper body system Justin into Rollin’ Justin, a fully self-sustaining mobile research platform.


In 2006 the humanoid upper body Justin was presented as a research platform for dual arm manipulation. Using its two arms and hands, controlled as hardware in the loop system, some basic control schemes for robust dual handed dual armed manipulation tasks have been evaluated and published. To further enhance the system’s field of work it was soon considered to equip it with a mobile base. This would allow using Justin also as a platform for mobile manipulation to study strategies utilizing the increased Fig. 1: The Rollin’ Justin workspace and redundancy for manipulation tasks. Also a The mobile base is intended as an enhancement of the mobile Justin serves perfect as a service robotic prototype to system and should not restrain the capabilities of the upper demonstrate enhanced autonomy features. This paper shows body. The large workspace and workload as well as the high the design considerations we took into account to build dynamic properties of Justin for example should not be Justin´s mobile platform as well as some of the technical limited by the platform. This imposes high demands on the details of the realized robotic system. static or dynamic stability of the base. On the other hand the II. BASIC DESIGN CONSIDERATIONS platform should be designed as compact as possible to pass through standard doorways and to make the appearance of Obviously the most adequate enhancement for “Justin” the whole system pleasant and nonhazardous. To decrease would be legs to make him walk and therefore a full system complexity, cost and project duration standard humanoid platform. However human inspired legs will components are to be used. The most preferred way would drastically increase the systems complexity adding at least be to integrate the system on an available mobile platform 10 to 12 degrees of freedom (DOF) to the already existing that can fulfill the functional needs. 43 DOF of the upper body. Also it would probably turn the A. Functional needs and available platforms research attention too much to walking and dynamical The upper body system itself has the mass, payload and stability issues than to the enhancement of the dual handed power consumption properties as listed in table 1. The manipulation capabilities. Therefore only wheeled solutions platform must be able to preserve active or passive (static) for the mobile base where considered. stability to the system if Justin lifts objects up to 20 kg from The basic design considerations ordered by their the floor. The power supply must provide up to 1 kW for the importance can be summarized as follows: actuation system and at least two computers for control and (1) Fulfill functional needs of the upper body perception of the upper body. For the convenient usage of (2) Prefer compact construction the base as workspace extension during manipulation tasks, (3) Usage of off-the-shelf components e.g. in front of a desk, the mobile drive system should be at least omnidirectional. To reach the best performance in the structure and its main components. The electronics power control of the total system the mobile base should allow supply devices are shown in Sec. IV followed by the closed loop control together with torso and arms within a communication concept (V) and some considerations on the bandwidth of 100 Hz minimum in hard realtime. platform control (VI). A conclusion is given in Sec. 0.

TABLE 1 III. MECHANICAL DESIGN Component Mass Payload Power Cons. Hand 1.8 kg 3 kg (Tip) 15 - 50 W The two-arm system Justin is a powerful upper body Arm 13 kg 15 kg (static) 50 - 150 W that is able to lift weights up to 20 kg. Its Torso+Head 15 kg 50 kg (static) 100 - 250 W arm span is 3000 mm and the torso can move about 600 mm Total ~ 50 kg ~ 20 kg (static) 0.5 - 1 kW to the front and 300 mm to the back [9]. The whole upper body can rotate 170 degree to both sides relative to its base. An intensive review of existing and commercial According to its weight (about 50 kg), workspace and force available platforms [1] showed that most mobile robots are this robot has to be mounted on a very stable base with an based on a statically stable platform principle. This means adequate footprint to prevent the robot from falling over. that no control or force/torque input is needed for The necessary footprint (maximum supporting polygon) maintaining stability. Within this group of platforms was evaluated by analyzing different torso positions (arms different wheels or tracks for propulsion are used. The vertical or horizontal oriented) carrying a payload of 3kg and number of legs equipped with wheels varies from two with at different accelerations [1]. The torques applied on the last one passive wheel (eg. Pioneer P3-DX, MetraLabs SCITOS axis of the torso ranged from 248 Nm in the static case up to G5), to be statically stable, over four (Pioneer P3-AT) and 1800 Nm. In any of these cases the mass and support six [2], usually used for off terrain vehicles, up to eight [3] polygon of the base should prevent tumbling down the wheels. It should be noted that vehicles with more than three system. wheels are hyper static and require some form of suspension The minimal footprint can be evaluated in a similar way if all wheels are to maintain good ground contact. Not many assuming a compact configuration of Justin in the static case. of the previously designed smaller platforms can carry Practically the minimal size of the platform is determined by higher loads as required for Justin except of some platforms the volume needed for the power supply, electronics and dedicated directly to carry upper bodies or industrial robot computer components to make it a fully autonomous system. arms like the Hermes [4], the ARMAR platform [5], the The wheel extension mechanism is designed to be a Twendy-One [6] or the Nomadic XR4000 [7]. Actually none very compact construction with highly integrated of them are commercial available. components. Due to the actuation by the wheels of the Recently the dynamically stabilized two wheeled platform themselves, no extra motors are necessary for the platform principle well known from the Segway RMP has extension and retraction function. been used for the integration of some manipulators like the NASA [8] or the Toyota Work Partner Robot. An advantage of such a system is the small weight and footprint of the platform but the power consumption and complexity is higher, safety issues in case of control failures are to be tracked and for manipulation actions the most critical factor is that the platform reacts negatively on stiff contacts with fixed objects in the environment. It can be stated that no off-the-shelf concept for a Fig. 2: Platform with extended and retracted wheels suitable mobile platform for Justin existed. In consequence a Fig. 2 shows the DLR mobile platform with extended new platform has been developed. and retracted wheels. It has a maximum length of 1220 mm B. The Platform Concept for Justin and a maximum width of 1052 mm. The minimum To keep the design of the platform as simple as possible dimensions are 812 mm x 644 mm. As the platform is a static base with four wheels was considered. To meet the equipped with spring damped wheel suspensions its height stability requirements during the manipulation tasks with varies between 658 mm and 728 mm. The weight is about high load and high dynamics as well as the need for compact 150 kg. design to move through narrow passages a variable footprint With extended wheels the platform has a footprint of mechanism was envisaged [1]. To allow easy integration of 985 mm x 815 mm. The decisive factor for the maximum the platform kinematics the base should support footprint is the stability. Together with a very low center of omnidirectional movements. Further the power supply of the gravity (about 260 mm above the ground quite centered to mobile base must also serve the upper body. The necessary the footprint) this footprint is enough to avoid a tilt over computation platforms and communication equipment have even in case of an emergency stop at full speed with the to be integrated as well. The result of the development of a upper body bent forward [1]. By retraction of the wheels mobile base for Justin is shown in Fig. 1. towards the platform the footprint can be reduced to In the following we introduce Justin’s mobile platform 685 mm x 515 mm (cf. Fig. 3). Here the decisive factor is the and its technical details. Section III describes the mechanical possibility to pass doors. While the wheels are retracted, the platform is able to go to every place where a wheel chair can A single-sided fork attaches the wheel to the steering go as well. drive. The steering drive is a type FHAC-mini motor- gearbox unit from Harmonic Drive, consisting of a brushless DC motor and a 100-fold reducing harmonic drive gearbox. The maximum steering speed is 360 °/s and the maximum torque is 28 Nm. The FHAC-mini is a high integrated off- the-shelf drive unit that meets the requirements on torque and velocity demanded on base of experiments with the mobile platform from the DLR project Robutler [10]. A type Whistle digital servo-drive controller from Elmo Motion Control is driving the steering motor. It is located next to the steering drive together with the filter and interface electronics of the motor controller. An additional Fig. 3: Maximum and minimum footprint type RE22S absolute encoder from Renishaw provides the of the platform absolute steering position at the output of the gearbox. With this mechanism to adjust the footprint the B. Wheel Suspension Rollin’ Justin can do fast or heavy manipulation and high Each of the four wheel units, consisting of hub motor, speed driving but nevertheless remains useful for indoor steering drive, servo-drive controller and absolute encoder, locomotion. is mounted on a special suspension: A parallel mechanism In order to obtain a stable stand with four wheels on the with a spring damper element. Fig. 5 shows the cross section ground especially on uneven terrain, the wheel suspensions of an entire leg of the platform consisting of the wheel unit, are individually equipped with a spring damper system. In the suspension with parallel mechanism and spring damper this way, also small obstacles like doorsteps can be passed in unit and the leg carrier with attached guide rails. The a gentle way. The height of the platform in a neutral state of retracted position is shown up in black outlines. the upper body is 693 mm. The spring can be deflected 35 mm around its equilibrium. leg carrier with A. Wheels and Steering guide rails The platform is driven by four independent hub motors (cf. Fig. 4). These are identical brushless-DC direct drives force sensor with a maximum torque of 30 Nm. They are driving a steel rim with thin (5 mm) solid rubber tires that have a diameter spring damper element of 216 mm. With this combination the platform can reach a velocity of 1.5 m/s (5.4 km/h). Due to the diameter of the leg lock tires and the torque of the wheel motors the Rollin’ Justin (over all weight: 199 kg) can pass steps up to 40 mm and climb ramps up to a slope of 17 degrees. The advantages of hub motors are firstly that the space inside the wheels can be used and thus the demand for a compact design is supported, and secondly the center of gravity is lowered due to the weight of the motors that is located near the ground. This supports the demand for stability.

damper lock encoder servo-drive suspension base controller steering drive Fig. 5 Cross section of the leg with wheel, steering motor, parallel kinematics and fork spring damper element The parallel mechanism enables the wheel to move in and out in horizontal direction without moving in vertical hub motor direction. The result is that the platform can vary its footprint while height and inclination remain unchanged. The mechanism is passive and actuated by appropriate wheel motion. To avoid unintentional extension or retraction of the

wheel the parallel mechanism can be locked (leg lock). A Fig. 4: Cross section of the wheel unit small servo motor that is programmed for two different positions opens and closes the lock of a toothed rack that nearly every electronics component inside the platform can connects the suspension base with the slider of the parallel be changed independently from other components. This mechanism. Slider and base are both mounted via linear simplifies maintenance work. bearings on guide rails which are attached to the leg carrier. The inside of the chassis is filled with the battery, the Due to the mounting of the suspension base on the same power supply, several computers, the hardware infrastructure guide rails as the slider of the parallel mechanism the entire and a user interface for the main controls of the system wheel unit can move up and down the leg carrier. A spring (cf. Fig. 7). The available space for the hardware is about damper element is connecting the suspension base to the leg 50 liters. carrier. The travel of the damper between its hardware limits is 70 mm. They are experimentally tuned to get the best performance in passing doorsteps and stabilizing the platform. The spring has a length of 170 mm, a spring constant of 5.47 N/mm and a pre-load of 24 mm. So the balance point of the four spring damper units carrying the Rollin’ Justin is nearly in the middle of the damper travel and the platform can pass steps up to 35 mm without reaching the hardware limits. The used dampers, type K0N9KX2-070 from Bansbach, are provided with a locking function. Another small servo motor releases or locks the damper by pushing or releasing a button on top of the piston rod. With locked dampers the platform behaves like a rigid base with high stiffness for Justin. This is recommended for manipulation tasks [11]. With released dampers the platform behaves like a car. It always has a stable stand with four wheels on the ground, can pass small steps and pitches slightly with changes of the center of gravity or acceleration. Between the damper and the fixing to the leg carrier Fig. 7: The platform entirely filled there is a force sensor in each leg with a range of 0 – with electronics 1000 N. By measuring the weight on each wheel it is possible to determine the center of gravity. Together with an inertia unit in the head of Justin it can be tried to identify the IV. ELECTRONICS AND SUPPLY stiffness parameters of the locked or unlocked platform, and a compensation as realized for the light-weight arms and When constructing a one of the most torso can be implemented. difficult tasks is to integrate the entire hardware for C. Platform Body controlling the robot and especially the power supply All four legs of the platform are attached to a frame that onboard. Many experimental mobile robots still have a is the basic part of the platform. The frame together with the connection to the environment via cable either for power legs builds the chassis of the platform that is shown in Fig. 6. supply or for realtime or high-speed communication [6] [12] [13]. But true mobility can only be reached when a robot can act completely autonomously without physical connection to a fixed place. So the mobile platform for Justin has to carry a proper power supply, four different computers for the control of Justin and the platform, all the infrastructure like network switches for the computers, a WLAN connection bridge, a radio receiver for emergency stop and of course the electronics of the platform itself like sensors, motor drives, realtime bus coupler, cameras, etc. A. Sensors As the platform should be able to localize and navigate itself safely in an unstructured environment, several sensors are needed that collect the required data. Besides the

navigation based on odometry data from the wheel Fig. 6 Platform chassis, movement (absolute encoders for the steering angles, body frame (blue) with mounted legs velocity of the wheel, absolute encoders for the wheel Apart from the function as leg carrier, the frame is also extensions) the platform has two video cameras for visual the carrier for every other part of the platform. The result is a odometry. One camera faces to the front of the platform, the very modular assembly of the whole vehicle. Every leg and other one to the left side. Based on the changes in the captured scenes per time increment, also called optical flow, the movement of the platform can be calculated. The result 2. Supply circuit is a redundant odometry with minimized error. The battery voltage fluctuates with the charge condition Simultaneous localization and mapping (SLAM) and between 40 V and 54 V. For the different robot components collision prevention can be performed using the data of this main supply has to be converted into proper voltage photo mixer device (PMD) cameras. These cameras provide levels. Six separated power lines are providing all the a depth value for every captured pixel in the scene. This components at their individual specified voltage level (48 V, delivers a 3D view of the environment around the platform. 3x 24 V, 12 V, 5 V). To cope with short circuits and over Four PMD cameras are observing the four main directions current errors every power line is fused at least once. For front, left, right and back. In this configuration the platform analysis and check up purposes the current is measured is able to detect walls, doors, obstacles as well as dynamical separately in every supply line and shown at a status display. objects like human legs. The supply circuit can handle a maximum (continuous) power of 3.7 kW. C. Emergency Stop, Computers and Peripherals As the mobile platform together with Justin is meant to be autonomous, it must not have any cable connection. To be able to stop the system in case of error anyway, a wireless emergency stop concept is required. A radio and infrared receiver on the platform controls a current loop that delivers an “OK”-signal to the relevant components (motor controllers and supplies) of the robot. As long as the receiver gets a coded message of the transmitter unit, the “OK”-signal is true. To be robust against Fig. 8: Field of view of the video cameras (left) disturbances the transmitter is sending on two different radio and the PMD-cameras (right) frequencies (433MHz and 868MHz) and via infra red. It can For additional safety issues every wheel has bumper be turned on and off by an emergency button. The timeout of switches to the forward and backward direction of the wheel. the receiver is set to 200 ms. Since the wheels are the outer parts of the platform a There are four MiniITX standard PCs with dual core physical contact with an obstacle will always be at the processors on board of the platform for control, bumper plains. The area between the legs is supervised by communication and peripheral I/O. The computers are the PMD-cameras. If one of the bumpers gets pushed, the connected to each other via Gigabit-Ethernet. The Ethernet platform can react by stopping immediately. But this is just a connection is done by an 8-port gigabit switch and the uplink safety option as in normal case the platform should detect to the control interface outside the platform is done by a and avoid all obstacles. Wireless-LAN bridge. The video cameras and the PMD cameras are attached via a second 8-port gigabit switch. The list of electronics on board the platform is B. Power Supply completed by the peripherals like firewire hubs, additional The power supply consists of a rechargeable battery as small supply units, USB controller for the user interface, energy storage and the supply circuit to provide the different different antennas for WLAN and emergency stop receiver, components with a proper voltage level. It is one of the most analog-digital converters for the force sensors, SSI- important hardware parts that has to be very reliable and interfaces for the absolute encoders, controller electronics failsafe. for the locking servos, several fans for cooling the interior, etc. 1. Battery The battery is a 48 V nominal voltage Lithium-Polymer V. COMMUNICATION accumulator that has a capacity of 40 Ah. The maximum discharge current is 200 A and the maximum charging The second important design guideline building the current is 80 A. The battery is made up of 13 cells in series platform was a compact construction and the third using off- with 3.7 V nominal voltage each. With an average power the-shelf components. Therefore the components were consumption of the entire Rollin’ Justin of about 600W the chosen upon their size and suitability for the functional battery is able to provide the system for about three hours. needs but not on the homogeneity of bus and communication The recharge can be done within one hour at maximum protocols. This requires the communication concept to cope charging current. with many different realtime protocols and bus systems. A balancing and safety electronic is monitoring and The hub motor wheels are communicating via correcting the operating parameters (voltage, current, CAN 2.0B at a command rate of 16 ms. The digital servo temperature) of each cell. Due to the very high energy drivers (Whistle) for the steering motors are using CANopen density of Lithium-Polymer cells the battery, together with (4 ms). The absolute encoders for the steering angle and the its monitoring electronics and a protective cover of stainless leg extension deliver their values via an SSI connection and steel, takes just 13 liters of the available space inside the the values from the force sensors have to be read by an platform. analog-digital-converter. The position of the servo drives for the locking functions in the wheel suspensions can be Besides the realtime part there are also components that commanded by a digital output signal. Arms, torso and head do not need to communicate in realtime. Video cameras, of Justin are using the realtime bus SERCOS and the hands PMD cameras, inclination sensor and status display are use HIC (Heterogeneous Interconnect, IEEE 1355). The connected to the standard I/O interfaces (Ethernet, USB, whole upper body system has a control loop rate of IEEE 1394) of the Linux computer. The Linux PC is also the 1 kHz [9]. gateway to the control hosts outside the platform attached Similar to the Justin control concept all the sensor data via WLAN interface. should be available in hard realtime on a single PC where the robot can be used within a Simulink environment as VI. PLATFORM CONTROL hardware in the loop system. Normally this would require a lot of driver programming, several PCI slots and cards for Mobile platforms equipped with several steering wheels the different bus systems and a lot of CPU power consumed are known to be omnidirectional, i.e., able to independently by data acquisition, which makes the design of the control translate and rotate on the plane [14]. As explained in system difficult and error-prone. Sec. III, Justin's mobile platform also possesses the ability to For Rollin’ Justin a commercially available PLC vary its footprint over time by extending or retracting the communication concept from Beckhoff is used. A single PC wheels, also during motion of the platform. Therefore, we is dedicated as data collector platform which retrieves the aimed at obtaining a control algorithm able to track an data from the CAN 2.0b, the OpenCAN, the analog, the SSI arbitrary (linear/angular) planar trajectory while, at the same and the SERCOS PLC components and transfers them in time, imposing an independent and decoupled extending or realtime to an EtherCAT bus. The data collector acts as retracting motion to each leg. This is achieved in two steps: EtherCAT Master. All the timing constraints and data Firstly, a suitable kinematic model representing the conversions are configured using the standard software special leg actuation mechanism is derived. Then, upon this TwinCAT from Beckhoff. On the dedicated control PC for model, a tracking controller is designed able to realize the the platform and torso control only an Ethercat Slave PCI proposed task by properly actuating the rolling and steering card is present, providing all sensor data of the entire system velocities of the wheels. A nice property of this feedback is via mapped memory. Therefore the communication that it dynamically linearizes the model equations, so that overhead on the realtime control PC is small and there is decoupled and exponential convergence for the tracking only one fairly simple driver for the EtherCat slave which error can be easily obtained. A full description of both has to be programmed. On the data collector PC however, modeling and control issues, together with an illustration of the communication overhead is quite high but there an experimental results, can be found in [15]. industry approved system is used. No driver has to be programmed. Only PLC components to start and stop the VII. CONCLUSION communication system have to be configured, which is also This paper presents the design and realization of Rollin’ sometimes a challenge, but less work intensive than Justin’s mobile platform. The mechanical assembly with programming the protocol drivers itself. Fig. 9 gives an parallel mechanism, spring-damper unit, hub motor, steering overview about the realtime communication concept of the drive and platform body is described as well as the structure Rollin’ Justin. of the electronics with the different sensors, the supply circuit, the computers and the battery. A novel way of integrating mechatronical components using several different realtime busses and protocols is shortly presented. With Rollin’ Justin a valuable robotic system is available for research on sophisticated mobile two-handed manipulation. This enables the development of strategies for complex manipulation or fetch and delivery tasks. These are typical demands for e.g. future service robotic applications done by humanoid robots.


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